This invention relates to an earth station for carrying out communication in a satellite communication system through a single satellite.
As a satellite communication system, a demand assigned multiple access system is well known in the art. The demand assigned multiple access system comprises a satellite, a control station, and first and second earth stations each of which includes first and second terminals. The first and the second earth stations are communicable with each other through a radio transmission path as first and second communication channels of first and second frequency bands which are assigned by the control station in the following manner. The first earth station transmits a connection demand signal to the control station through the satellite by using a common control channel of a common frequency band when the first terminal produces a transmission request signal in order to communicate with the second terminal of the second earth station. On reception of the connection demand signal, the control station transmits a first channel assignment signal representative of the first communication channel to the first earth station by using the common control channel. Responsive to the first channel assignment signal, the first earth station transmits a first transmission data signal through the first communication channel towards the satellite. On the other hand, the control station transmits a second channel assignment signal representative of the second communication channel to the second earth station by using the common control channel in order to prevent a collision of signals. Responsive to the second channel assignment signal, the second earth station transmits a second transmission data signal as a response transmission data signal through the second communication channel towards the satellite. Thus, the first and the second earth stations are communicable with one another.
In order to transmit the connection demand signal and to receive the first and the second channel assignment signals, each of the first and the second earth stations requires a transmitter and a receiver for the common control channel. It is to be noted that the demand assigned multiple access system becomes expensive and complex in structure because the control station should be included in the demand assigned multiple access system. Furthermore, the demand assigned multiple access system has a reduced availability of the radio transmission path because the common control channel should be used for the connection demand signal and the first and the second channel assignment signals. It should furthermore be noted that one of the first and the second earth stations can be called afresh as a first earth station without loss of generality.